The invention relates to a hydraulic valve arrangement with locking function, with a control valve, which in two operating positions connects one motor connection with a pump connection and a second motor connection with a tank connection and vice versa, and in a locking position separates both motor connections from pump and tank connection, and with two lock valves, each connected between the control valve and one of the motor connections.
Known hydraulic valve arrangements of this kind are mainly, but not exclusively, used in tractors, which can be provided with auxiliary tools, like for example snow clearance vehicles or street cleaning vehicles. The lock valves have a piston loaded by a spring and limiting a spring chamber, the spring chambers being pressure releasable. The pressure release of the spring chambers, which is required to hold the lock valves open in the operating positions and in the floating position of the control valve, occurs in that release channels end in the bore of the control valve and their end is overruled by the slide of the control valve. In certain cases, however, it has turned out that errors occur in the function, particularly in the locking function.
The task of the invention is to provide a hydraulic valve arrangement as mentioned in the introduction, which has a better operational behaviour.
According to the invention, this task is solved in that for each lock valve a pressure release valve is provided, whose operating member is loadable in the closing direction by a first chamber pressure acting in a first chamber and an additional force and in the opening direction by a control pressure acting in a second chamber, and that the control pressure in dependence of the position of the control valve has a lower value, which is equal to the first chamber pressure, and an upper value, which exceeds the first chamber value by such a value that the oppositely acting closing force is overcome.
The application of pressure controlled pressure release valves gives a large freedom in dimensioning, which is advantageous with regard to space utilisation and the solving of leakage problems. The dependence of the control pressure on the first chamber pressure ensures that variations in this pressure, which, particularly in large systems, cannot be avoided, have no influence on the mode of operation of the pressure release valves. This gives a high degree of operational security.
This is particularly the case when the first chamber pressure is equal to the tank pressure.
It is advantageous that the upper value of the control pressure exceeds the first chamber pressure by a constant value. This enables a particularly safe pressure release.
With regard to design, it is recommended that the pressure release valves are lifting valves and their operating elements are pistons. Such lifting valves with pistons can easily be dimensioned so that in the closed position they are completely tight.
Favourable is a pressure source producing the upper value of the control pressure, said source being separated from or connected with the second chamber by means of the control valve, and an blow-off throttle between the first and the second chambers. A connection will produce the upper value of the control pressure by means of the pressure source, a separation will produce the lower value of the control pressure by means of the blow-off throttle.
It is advantageous that the pressure source producing the upper value of the control pressure is formed by the outlet of a pressure control valve fed by the pump pressure, said valve being loaded in the closing direction by the outlet pressure and in the opening direction by the tank pressure and a spring. Such a pressure source is easy to make and can without problems be arranged in a hydraulic arrangement.
It is recommended that the pressure source producing the upper value of the control pressure also produces the pilot pressure for the electrical activation of the control valve. Thus the pressure source is multiply useful.
In a preferred embodiment it is ensured that a load pressure sensing system is provided, which is independent of the control pressure, and which is connected with. the tank in the locking position of the control valve. The clear separation of control pressure and load pressure enables the provision, in the locking position, of the known and desired connection of the load pressure sensing system with the tank, without having to put up with disadvantages in the control pressure.
Further, it is advantageous that in one operating position the control valve connects the pressure source producing the upper value of the control pressure with the second chamber of the first pressure release valve and the other operating position connects it with the second chamber of the second pressure release valve. The control valve therefore also secures the correct direction of the inlet of control pressure to the belonging pressure release valve.
It is also advantageous that additionally to the two operating positions and the locking position, the control valve has a floating position, in which the second chambers of both pressure release valves are connected with the pressure source producing the upper value of the control pressure. The valve arrangement can therefore additionally also be equipped with a floating position, in which both lock valves are kept open.